ajpa 1980 a quatitative comparison of the hominoid thalamus 3 motor

8/14/2019 AJPA 1980 a Quatitative Comparison of the Hominoid Thalamus 3 Motor

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AMERICAN JOURNAL OF PHYSICAL ANTHROP OLOGY 52405-419 (1980)

A Quantitative Comparison of the Hominoid Thalamus:I l l . A Motor Su bs t ra te the Ventrolateral Complex

ESTE ARMSTRONGDepar t men t of Anthropology, Colurnb~ciUniwrsi t .y , New Y o rk , N e w Yo rk, cindDepar t men t o f A nci t omy , L ou is i ana S t a t e Uni uers s ty , N ew Orleons,Louiaicina 70119KEY WORDS Thal amus , Ventrolateral complex, Evolution

ABSTRACT Nuclear volumes, nerve cell densit ies, numbers of neurons, andvolumes of nerve cell perikarya of the thalamic ventrolateral complex tVL), aneural subst rat e for movement, were measured in specimens from two gibbons, onegorilla, one chimpanzee, and t hre e humans, an d th e values were compared. Thehuman VL had about one-and-a-half times as many neurons as did those of thegreat apes. The relative frequencies of the sizes of nerve cell perikarya differedslightly i n th e ventro lateral segment of VL; no differences were noted in th e rest ofVL. Compared with findings from other pa rts of the tha lamus , the differences inthe volumes of VL were grea ter t han those found in th e thalamic sensory nuclei,similar to those of the rest of the thal amus , and less tha n those found in th e wholebrain. The increased number of neurons in human V L was similar to tha t of thesomatosensory relay complex, but g reater than those of th e auditory and visualnuclei and less than those of the limbic and association nuclei. In huma n evolution,the numbers of neurons in the VL appeared to increase at a faster rate th an didneurons of the pyramidal tr act , whereas the motor cortex apparently increased at arate greater than VL

During hominid evolution, bipedalism andthe oral and manual dexterity necessary forlanguage, facial expression, and tool-makingmust have been accompanied by alt erat ion inpart s of the neural subs trat es for movement,(Holloway, 75, '70, '68;Phillips and Porter, 77;'I'uttle et al., '79).Some of the motor regions' ofthe brain among hominoids have been com-pared. The motor cortex and the cerebellum ar erelatively prominent in humans t Passingham,'75; Stephan et al., 70).Another motor region,the striatum, showsa small overall differentialenhancement (Step han, t al., 70;Stephan, 79)but possible species-specific shifts between itscomponents tBonin and Shariff, '51; Frahm etal., '79; Harmon and Carpenter, '50; Holloway,'681.The ventrola teral complex VL), th e specificmotor relay nucleus of the thalamus, is inter-posed between the pyramidal system of themotor cortex and t hr ee other motor centers ofthe brain. the cerebellum, the globus pallidus,and the substant ia n igra (Carpenter , 76;

Strick, 7 6 ) .The cerebellum and motor cortexproject to all parts of VL, whereas the globuspallidus and substantia nigra have more lim-itedconnections (Mehler et al.,'58; Mehler, 71,Rinvik, '75; Kunzle, '76; Catsman-Berrevoetsand Kuypers, 7 8 ) .Nonmotor cortical inp uts a swell as int er nal (e. g., cortico-cerebello-thalamic-cortical) and external motor feedbacksystems pass through VL (Phillips and P orte r,77) . Part of the reticular activating systemth at desynchronizes the cortical electroenceph-alogram is relayed through VL (Purpura, 72,Ohmoto et al. , '78). Integration among t he in-coming system s uses populations of int er-neurons (Desiraju and Purpura, '70; Purpura,72) as well as differing synaptic patte rns onrelay neurons tDeniau et al., 7 8 ) .'The t e r m motor region.; 01 the 1)r:iin doe5 not rnciin t h a t t h e s e

dieas i i i i n v o l \ e d only in m u v r m e n t 01 t h x t other :ire:ih h a ve n oi n v o l v e m e n t T h p m o t or ;ireas ii m o r e d i r e c t l y t i e d in w i t h m o v e m e n tt h a n are other iireiih, h u t t h e y d m par t ic ipa te i n o t h e r f u n c t i r m a lsyhtem.;Received J u n e 23 1979, a c c e p t e d Se p t e m h e r 13, 1979

0002-9483/8015203-04~5$0~0 (1 1980 ALAN R LISS, I N C 405


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406 ES I E A RMSTRO N GThe major output of VL is to the motor cortex,on which it can exert a direct and powerfulinfluence (Yoshida et al., '66; Strick and Ster -ling, '74; Phillips and Porter, 77).Activation ofVL before voluntary movement may representa general adjustment of posture to accommo-dat e the movement (Eva rts, '71; Joffrey andLamarre, 74; Jasper and Bertrand, '66). VLalso helps to control muscle tone and speed ofmovements t Hassler, 72; Jasper and Bertran d,'66).Activities of cells in VL are not res trictedto a particu lar phas e of movement such as flex-ion, but app ear correlated wit h complexmovements iMassion and Smith, 73; Massion,Tha t many of th e postural and phasic func-tions of VL have been found in different mam-

mals (E var ts, 71; Amassian et al., '72) does notpreclude species-specific adaptat ions. Posturaladjustments are l ikely to differ amongbrachiators, knuckle-walkers, and bipeds. Ad-aptations within VL to supporta human behav-ior pattern is suggested. Studies of natural le-sions from hemorrhages and tumors and fromstereotaxically placed lesions made to abolishtremor and rigidity in patients with Parkin-son's disease indicate that VL is involved inspeech (Fazio et al., 73; Ojemann, '76; Bell, '68;Riklan and Levita, '69). Much of the involve-ment is with pure ly motor aspects of languagesuch as voice volume, rate of speaking, andarticylation t Rikl an and Levita, '69). Whetherthe parts of VL involved with the motor aspectsof language are also involved with nonspeechoral motor activities t,Mateer, '78) is not yetknown. One of th e requisite mechanisms for thecontrol for speech, tha t of controlling respira-tion, may be located in VL tOjemann, '76). Al-terations in VL do not preclude changes inother parts of the brain tha t support the samefunction. Changes in the termination of thepyramidal tract neurons may also be a neces-sary respiratory substrate for speech produc-tion (Phillips and Porter, '77). In addition tothese motor activities, VL in humans appearsto be involved in more complex cognitive tasksin a n asymmetric fashion. Lesions in th e rightVL increase errors in visual perceptual per-formance l eg ., face matching), whereas lesionsin the left VL impai r verbally mediated cogni-tion tOjemann, '76; Vilkki, '78).

'76-'77).

M A T E R I AL S A N D M E T H O D SA brief description of the specimens andmethodology is given in this paper; details

ar e provided elsewhere (Armstrong, '79). Thespecimens included two sides of a gibbon brainof which the species, age, and sex were un-

known, and the brains from a male Hylobateslar th at was presumed to have lived in the wild,from a 10- o 12-year-old male chimpanzee anda 14-year-old male gorilla t ha t had spen t mostof the ir lives in zoos, and from three re lativelyyoung human adul ts, ages 19, 28, and 32. Thebody weights for th e gibbon ofunknown speciesand two human s (referred to as Homo-s and - t)were estimated at 5.5 kg, 136.1 kg, an d 49 kg,respectively. The others were measured at timeof death tH. lar, 5.9; chimpanzee, 63.5 kg;gorilla, 203 kg; Homo-c, 113.6 kg).Because the specimens came from differentlaboratories, some variations in the size of t hebrain due to different preparative techniquescould be expected. The use of relative or pro-portionate values for comparison minimizeddifferential processing effects. Relative vol-umes such as the percent of the whole thalam usoccupied by the nucleus and es tim ate s of thetotal number of neurons tdensity per volumeper total nucleus) ar e quantitative yet pro-portionate and thus suitable for comparativework. Estimate s of absolute volumes, neuronaldensity, and sizes of neuronal perikarya arealso recorded.The relat ive frequencies of the different sizesof nerve cell pe rikary a were compared. The dif-ferences between the gibbon brain whose twosides were cut horizontally or sagittally and theother hominoids whose brains were all sec-tioned coronally were compared only if bothsides of the gibbon brain resembled each other.Also, effects from sectioning or shrinkageshould be evident i n both nuclear segments ofVL (designated VLo and VLcm, as describedlater). A finding in one, but not both, nucleisuggests a biological difference.VL and its subdivisions were identified a c -cording to the appearance and arr ange ment ofneurons. Outlines of VL were traced throughserial sections. Nuclear volumes were deter-mined by summing the measured ar eas , multi-plying them by the tissue thickness and dis-tanc e between them, and dividing by the mag-nification (Bauchot, 63). Estim ates ofneuronaldensity were made from counting nucleoli.

Within specimen comparisons of neuronaldensities and perikaryal sizes used Student'stwo-tailed t test . Allometric analyses were doneby the least squares method. Step-wise regres-sion followed th e SPSS program and was usedto determine which variables best predicted thevolume of VL. Lack of number of specimensprecluded use of all t he variables te.g., volumeof motor cortex and dentate nucleus).Widely varied nomenclature describes thelateral tier of nuclei in the dorsal thalamus.


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HOMINOID VENTROLATERAL COMPLEX 407The names for the populations of nerve cells,nuclei, in this work and other papers (Arm-str ong, '76, '79a), are based on th e neu-roanatomical studies of Walker ('38) a n dOlszewski ('52).Observations of the macaquemonkey iM. rnulutta and to a lesser extent thechimpanzee tP. troglodytes determined theprimate paradigm in thi s nomenclature. Mostresearchers using experimental animals rec-ognize t hree divisions (pa rs oralis, medialis,and caudalis) of the ventrolateral nucleus.An al ternat ive terminology is der ivedprimarily from the work of Hassler ('59)andVan Buren an d Borke ( 7 2 ) .The nomenclatureand division come from studies on humans andare important in the clinical literature. Thesame general region of the later al thal amu s isdivided into nuclei ventro-oralis and dorso-oralis.A correspondence between the two ter-minologies has yet to be worked out. Theheterogeneous cytoarchitecture a nd poorly de-fined borders produce disagreement. Pa rt of thedifficulty has been a lack of evolutionary per-spective of the differences. The nuclear labels,even though they sound topographic, shouldnot be considered topographically accurate forany one species. The patterns exemplified inthe Nissl-stained sections of the present studyappeared to resemble the descriptions by Has-sler ('59) nd Van Buren an d Borke ('72). n allthe hominoids, however, I could identify a re-gion in the l ateral tha lamus between th e ven-troanterior nucleus and the ventrobasal com-plex or the latera l posterior nucleus. Followingth e nomenclature of Walker ( 38), called thisregion the ventrola teral tVL) complex.

Two groups of nuclei were consistently ob-served in th e VL region of the hominoids of th isstudy. One population of neurons was found inthe ventrolateral segment of VL. The neuronalarchitecture resembled tha t ofventro-oralis ex-ternus of Hassler ( 59) nd Van Buren andBorke ( 72) . ecause of the s trong cytoarchitec-tural agreement of the ventrolateral segmentofVL with ventro-oralis externus, I labelled th eregion ventrolaternl oral i s t VLoj. The ventro-latera l oralis discussed here is probably equiva-lent to the ventroventralis of Krieg ('48) andpossibly to pa rt of the ventrolateral medialis ofCarpenter ( 7 6 ) . combined the remainder ofVL and labelled it V L cauda l i s and rnedialistVLcm).

RESU LTSThe ventrola teral complex tVL) is located inthe lateral portion of th e dorsal thalamus. VL isbounded anteriorly by the ventroanterior nu-

cleus tVA) and posteriorly by the ventrobasalcomplex (VB ) and lateral-posterior nucle us(LP). The exte rnal and internal medullarylaminae are located laterally and medially(Fig. 1-3).The border between VA and VL was difficultto delineate, particularly in coronal sections.The more fr equ ent occurrence of fusiform cellsin VL (Van Buren and Borke, '72) was the bestcriterion for separa ting these two nuclei in thegrea t ape and human specimens. The neuronalmorphology of the gibbons was less differ-entiated. A lighter intensity of staining in VLwas the major criterion for separa ting VA fromVL in the gibbons. A difference in sta ini ng be-tween VA and VI, was not noticeable in th eother hominoids, but had been noted by othersin the gibbon (Kanagasuntheram and Wong,'68). The highe r densi ty of nerve cells in VLthan in VA aided in the separation iFig. 3 ) ,butthe variability within each nucleus was high.No significant differences in neuronal densitywere observed between VA and th e neighbor-ing part of VL, VLcm, in any of these specimenswhere neuronal densities were measured [sag -gittal a nd horizontal sectionsof the two sides ofthe gibbon brain (gibbon -sa nd -h ), = 0.25and0.52, respectively; gorilla, t = 1.6; human, t =0.251.The posterior border with VB was estab-lished by the presence of very large neurons inVB. The border with LP was defined by thelarger neurons and lower neuronal density ofVL. The differences in neuronal density werenot statistically significant ( in gibbons-s and-h, t = 1.07 and 0.83, respectively, gorilla, t =1.4;and human, t = 0.3).The two components of VL, which were stud-ied separately, were delineated on the basis ofthe arra ngement and staining characteristics

A h h r w i n t i o n sCC - corpus callosumCM - centromedianum nucleusILA - interlaminary nucleiLD - lateral dorsal nucleusLGB - lateral yeniculate bodyLP - lateral posterior nucleusMD - medial dorsal nucle usMGH - medial geniculate bodyMID - midline nucleiPC - paracentral nucleusR - reticular nucleusVA - ventroanterior nucleusVB - ventrohasal complexVLc ventrolateral nucleus, parsVLm - \entrolateral nucleus, parsVLo - ventrolateral nucleus, pars oralis

caudalismedialis


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408 ESTE ARMSTRONG

Fig. 1.Fig. 2.

Coronal section through the thalamic ventrolateral complex of the gorilla. NisslCoronal section through the chimpanzees ventrolateral complex. Nissl stain.stain.


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HOMINOID VENTKOLATEKAL COMPLEX 409

Fig. 3 . Horizontal section through gibbon thalamus. Niss l stain

of the nerve cells. In all of the hominoids, VLcmhad a more regular dispersion of neurons thandid VLo, whose neurons were clumped by cours-ing fibers iFig. 1 and 2 ; the arrangement wasnot correlated with neuronal density. Thehuman specimen was the only hominoid withVLo less densely packed than VLcm (Table 1 .This intraspecimen difference was statisticallysignificant ip 0.001, t = 30). Nerve cells inVLcm were more ir regular than those of VLo inthe ir take-up of Nissl stain . The neuronal peri-karya were slightly smaller in human VLothan VLcm, but slightly larger in the otherhominoids (Table 2 ) . The differences were notsignificant in humans, gorilla, or chimpanzee( t= 1.36,t = 1.36, and t = 0.25, respectively),butwere significant in gibbons (t = 5.36, p < 0.01).The volume of VL was correlated with differ-en t factors. A step-wise regression determinedthat in hominoids VL volume was best pre-dicted by brain weight (RL = 0.99475). Theaddition of neuronal density to brain weightdecreased the residuals further (R = 0.9984),and when the volume of the rest of thethalamus was added to the above two factors,

the predictive value became close to perfect R L= 1.00000. Body weight did not predict VL vol-ume as well as the above factors. The indepen-dent variables will be analyzed separately.The volume of VL is larger in the largerbrains (Table 3 Fig. 4). When log. VL is re-gressed aga ins t log. bra in weight (E), he slopeis 0.858 (withr = 0.99, p 0.001) and is signifi-cantly less than one ~p 0.01).A slope less than1shows tha t the rest of the brain h as expandeda t a greater r ate than has VL. The cortex is themost likely region for greater expansion(Stephanet al., '70).The part of the cortex with which VL hasreciprocal connections and, therefore, wherechanges ar e most likely to covary with VL isthe motor cortex. A s an initial analysis, I com-pared the differences I found in VL in a gibbon,chimpanzee, and human with those in corticalprecentral surface (are as 4 6) in the samespecies (Glezer, 68). Since there is a wide rangeof individual variation in regions of the brain(e.g. , Towe, '73; Armstrong, '801, these figuresmust be considered tentative. When the totalprecentral region is regressed against the total


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412 ESTIS A RMSTRO N G

Fig. 4 V ol ume of ventrolateml complex relative to brain weight

VL complex, th e slope produced is 1.008, with r= 0.989, t, = 6.76, p 0.05 iFig. 5).If the sur -face area of the cortex changed among thespecies at the same rate as the thalamic vol-ume, a slope as derived from least-squaresmethod would be abou t 0.67 (Huxley, 72; Je ri -son, 73).The slope found here differs signifi-cantly from 0, but while it is strongly sugges-tive tha t the slope is higher th an th e 0.67 slopeof equal development, the difference does notquite reach statistica l significance (t , = 2.29, p= 0.1).In addition t o it s reciprocal connections withthe motor cortex, VL receives fibers from thedent ate nucleus of the cerebellum. The denta tenucleus has been studied volumetrically inhumans, a gorilla, and a chimpanzee (Daron,'60; Fix, 67; Hopker, 51). If th e logarithm of VLvolume is analyze d as a function of thelogarithm of the volume of the dent ate nucleus,isometry would be represented by I. The slopeactually produced, 1.41, significantly differsfrom 0 (T, = 4.98, p < 0.05), but not from 1(t, = 1.44, .2 > p > .1). VL has increased at thesame or a slightly greater rate th an ha s thedent ate nucleus.The rat e of divergence in th e volume of VL isalso similar to tha t of th e rest of the th alam us.VL occupies about 13% n all hominoid thala mi(Table3).An allometric analysi s of the volumesby the least squares method produces a slope

close to the isometric value of 1 iFig. 6 ). Theslopes with and without h uman values a re close(1.055and 1.039, respectively, both r = 0.99).The two parts of VL have different rates ofchan ge. The gro wth coefficient is slig htlyhigher for VLo th an for VLcm (Fig.6, Table 3).The difference between the two components isthe result of hu ma n values in VLo (slope 1.12with hum an values and 1.03 without; r = 0.995both). More specimens must be analyzed beforea n evolutionary effect can be separated fromone due to individual variation.The volumes of VL were correlated wi th bodyweight ir = 0.85, p 0.05), Fig.7 ) .The correla-tion could be strengthened by restricting thecorrelation to apes ( r = 0.975, p < 0.01) or togibbons and humans ( r= 0.993, p p > 0.05). The


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HOMINOID VENTHOLAI'EKAL COMPLEX 413

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Fig. 5, Relationship between ar ea of precentral cortex (ar eas4 and 6 nd ventrolateral volume. Bars representSD of VL volume. G represents hypothetical position for gorilla. Slope is 1.008.

hu m an va lues i n VLo produced the l a rges t a l -tera t ion in the s lope (s lope of VLo wi th h um anvalues included = - 0.486 a n d w i t h o u t h u m a nva lues =-0.338, 0.1 > p > 0.05).The es tima ted num ber s of neurons a r e l i s tedi n T a b le 3. The grea t apes have about two-and-a -ha l f t imes as many motor neurons a s doth e g ibbons. The hu m an va lues a r e e s t ima teda t one-and-a -ha lf t imes as m a n y a s t h e g r e a tapes . For g ibbons , ch impanzees , and hu m an sthe num ber of pyramida l t r ac t neurons (Towe ,'73) corre la tes s t rongly w i th the nu m be r of VLn e u r o n s i r = 0.995, p < 0.01). W h e n t h elogar i thm of th e numb er of pyram idal t ra ctneurons i s regressed agains t the logar i thm ofth e number of VL neurons , t h e s lope i s 0.85.Th e l a rge r r a t e o f i nc rease i n num ber s of VLneurons i s probably because both integrat ivean d r e lay neurons were counted , whereas t h ees t imates of pyramida l t ract axon s involve onlyrelay neurons .The re la t ive s izes of the neu ronal per ikary aar e simi l a r i n a l l t h r e e spec imens (Tab le2 , Fig.8 a n d 9).T h e r e i s a s l igh t ly g rea t e r separa t i onof the taxonomic group s in VLo tha n in VLcm.

DISCUSSIONCompar ison of the thalamic speci f ic motorcomplex amon g ex tan t hominoids shows a ra teof divergence in t i ssue volume a nd nu m be r ofneurons that i s l e s s t han t ha t fo r t he wholebra in , bu t s imi l a r t o that of the res t of t h et h a l a m u s ( F i g . 4 a n d 6). T h e t h a l a m u s i s o n epa r t of the dienceph alon, and the lower ra t e ofVL and tha lamic divergence is concordant wi thth e re la t ively s low expansion of th e diencepha-

lon among pr im ates iS t ephan e t al., '70).The da t a a l so sugges t , however, t h a t d ur inghomin id evo lu ti on t he re w as a sma l l , but dif -f e ren t ia l expans ion i n VL . S ince t he a l t e r a t i on sin VL s volume ar e i sometr ic wi th th e res t oftha l amus , a f i r s t incl ination i s to thin k that t h echanges , being propor t ional , represent a s t a -bi l ization or a nonder ived condi tion. Nev er the-less, a more conservat ive s ta tu s was recognizedin a l l th e tha lam ic sensory nucle i, whose vol-umes va r i ed l e ss t h an t hose i n t he r es t of t het h a l a m u s ( A r m s tr o n g , '79). F u r t h e r m o r e , t h ev i sual and aud i to ry t ha lamic nuc l e i have a s im-i l a r numb er of neurons i n g rea t apes an d hu-m a n s , b u t V L h a s o n e - a n d - a - h a l f t i m e s as


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414 ESTE ARMS TRONG

Fig. 6. V ol ume of ven t r o l a t e r a l compl ex a nd nuc l e i as a funct ion of t h e r e s t of the th; i l; rmus. L ine 1. tota lvent rola tera l com plex; l ine 2 , VLcm; l ine :3 VLo.

many neurons in humans as is found in greatapes. The differences were less intense thanthose found in th e thal amic limbic and associa-tion regions (Armstrong, '80).Both th e degreeof volumetric change and number of neuronsargue for a n active selection in t he motor nucleiduring hominid evolution.Similar volumetric changes have been foundamong the different systems in the cortex.Human an d chimpanzee motor cortices appearvolumetrically more divergent t ha n do the vi-sual cortices (Shariff, 5 3 ) . t is unclear whetherthe association and motor cortices have a simi-lar or different degree of divergence among ex-tant hominoids (Holloway, '68; Passinghamand Et t inger , '74; Shariff, '53). Amonghominoids the motor cortex has expanded at afaster rate tha n VL (Fig.5).The bigger growthof th e motor cortex may represent a structural

corticalization of motor function Noback andMoskowitz, 63 ) . Whether. the corticalizationwould be better considered as a neural sub-strate for new behaviors ( Jerison, 731or for therefinement an d control of old ones (Noback andMoskowitz, 63) is presently unknown.A major output of the motor cortex, thepyramidal tract (PI'),has been measured inseveral hominoids (Towe, 73). The changes inthe numbers of neurons in VL and PT arehighly correlated, but the growth coefficientshows a larger relative increase in VL. Al-though some small axons may have been over-looked in the counts of PI xons (Towe, 73),there is no evidence that humans have moresmall axons tha n do apes in thi s long distancehard-wired trac t. Since the numbers of axons inF T have increased a t a slower rate than thenumber of VL neurons, but the motor cortex


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HOMINOID VENTROLATERAL COMPLEX 415

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1 0 1 0 0

R O D V WT.. K qVolum e of ventrolateral complex relative to body weight.ig. 7.

has increased a t a greater rate than h as VL, thestructural corticalization may likely be t he re -sul t of an increase in integrative nerve cells orprocesses or both. The motor cortex is composedof overlapping sub are as tha t project to specifictargets via PT (Phill ipsand Porter, 77) , nd anincrease in ti ssue not directly tied to P T wouldpermit increased combinations and recombi-nations of motor programs. Such variation inmotor programs may underlie skill and dexter-ity.Controlled tnovement requires continualadjustments. VL nerve cells send signals to th emotor cortex that have been generated by ex-ternal and i ntern al feedback networks (Phi l-lips and Porter, 77) . The cerebellum is a majorsource of such feedback. The cerebellum hasexpanded among prim ates at a ra te second onlyto the neocortex and certainly a t a greater ratethan that of the diencephalon (Steph an et al.,'70). Nevertheless, the major relay nucleus ofth e cerebellum, t he de ntat e nucleus, appears tohav e increased in volume only as much a s VL or

perhaps slightly less. That finding is not a dis-crepancy, because the dentate nucleus is arelay center, whereas VL is an in tegr ativ e one.The expansion in VL may be an example ofsubcorticalization (Noback Moskowitz, '63)and suggests that for a given function, integra-tive centers are likely to change more thanrelay centers.The above analyses do not account for themost clearly observed differences in motor be-havior among ex tant apes, tha t of locomotion.The nucleus VLo may be a region to explore inthis regard. The hominoid VLo occupies thetopographic region tha t in cats is involved withlimb muscles and postural adjustments (Mas-sion, 76- 77; Smith et al. , '78). More perti-nently, shift s in volume and sizes of neuronalperikarya among these specimens suggestspecies-specific differences in VLo, but not inVLcm. The human VLo manifests a bigger rel-ative increase in volume accompanied by aslower addition of nerve cells than does VLcm(Fig.6, Table 3). Changes of this nature might


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416 ESTE ARMSTRONG

37 5c .

Neuron Volumes in mlcra'L O

Human -Gorilla -

Fig. 8. Comparison of the relative frequencies of nerve cell perikaryal sizes in VLcm


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HOMINOID VENTROLATEKAL COMP LEX 417

2 4 7v

HumanGorillaGibbon O O

~

Y . . . .800 I600 3200 4800 b4 8000 ,8000Neuron Volumes in micrajV L c r n

Fig. 9. Comparison of the relative frequencies of nerve ce l l perikaryal sizes in VLo.


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418 ESTE ARMSTRONGsignal a different organization of neurons. Onthe other hand, fibers passing through VLo(Fig.1and 2 leaving from and terminating inother parts of the brain could, via an increase infiber number and/or size, create such a condi-tion without active selection working in VLo.Arelative decrease of larger neurons in the gib-bon VLo, but not VLcm (Fig. 8 and 9), alsosupports a separation of locomotory groups inthis nucleus. Further comparative studies arewarranted.

ACKNOWLEDGMENTSI thank C.R. Noback, H. Stephan, N. Mos-

kowitz, W.I. Welker, The Max-Planck-Institutfur Hirnforschung, Frankfurt, and the Yakov-lev Collection for the opportunity to study someof their primate brains; R. Holloway, C.R.Noback, and W.I. Welker for their technicaladvice; T. Hill for the excellent photographs ofthe chimpanzee brain; and S. Schmidt and L.Steiner for the typing of this manuscript. Thiswork was supported in part by NSF SOC. rant74 20149 0113.24 019.00 and NINDS grantsNB-3249 and NB-06225 from the USPHS.

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